Method of priming a metal surface with a terpolymer solution containing a polyhydroxy compound



United States Patent 3,285,772 METHOD OF PRIMING A METAL SURFACE WITH ATERPOLYMER SOLUTION CONTAINING A POLYHYDROXY COMPOUND Ulrich Rietz andPeter Schmitz, Munich, Germany, and Hermann Unterguggenberger, deceased,late of Munich, Germany, by Louise Unterguggenberger, RolandUnterguggenberger, and Christa Unterguggenberger, heirs, all of Munich,Germany, assignors to Wacker-Chemie G.m.b.H., Munich, Germany, a firm ofGermany No Drawing. Original application Nov. 10, 1960, Ser. No. 68,367.Divided and this application Dec. 16, 1964, Ser. No. 419,629 Claimspriority, application Germany, Dec. 30, 1959, W 26,981 6 Claims. (Cl.117-75) The present application is a division of copending applicationSerial No. 68,367, filed November 10, 1960.

It is a common practice to coat metal surfaces with polyvinyl chlorideand other film-forming resins in order to protect them, for example,from corrosion or to render them chemically resistant. In order toobtain a good bonding of the coating composition to the metal surfacebeing protected it is essential that a suitably adherent base or primecoat first be applied to the metal surface. Base coating or primingcompositions which have been found to be especially suitable for thispurpose are those in which the film-forming component is a copolymer ofvinyl chloride and vinyl acetate modified with an unsaturateddicarboxylic acid. The final coating composition or finish is thenapplied by spraying, dipping, roller coating or brushing and in the caseof plastisols or organosols comprising a vinyl polymer or copolymer thecoated metal surface is then baked to cause the vinyl polymer orcopolymer to fuse and to produce a suitably adherent coating.

The above base coating or priming compositions are not suitable,however, for use in the priming or undercoating of metals such as zincand its alloys, as well as iron and tin plate, which accelerate thethermal decomposition of the vinyl resins and impair the bond. In orderto minimize any decomposition of the primer or undercoat in the coatingof active surfaces such as zinc and zinc alloy material, for example, ithas been the practice either to plate the latter with copper or tophosphatize the surface, or else to control the baking temperature andto maintain the temperature of the subsequently applied surface coatingcompositions below about 150 C. These methods have proven to beunsatisfactory, however, since they are not only troublesome but inorder to produce desirably glossy surface coatings with those coatingcompositions having a basis of polyvinyl chloride a baking temperatureabove 180 C. is necessary. Even with the addition of appreciable amountsof the commonly employed inorganic and organic stabilizing agents littlenoticeable improvement is obtained. Mixed copolymers of vinyl chlorideand vinyl acetate modified with maleic acid as well as those containingpolycondensate resins while giving excellent adhesion do not permit theuse of the higher baking temperatures which are necessary in order toproduce coated surfaces exhibiting a high gloss.

It is, therefore, an important object of this invention to provide a newand improved method of coating zinc and zinc-alloy surfaces by primingand undercoating said surfaces with compositions comprising modifiedcopolymers of vinyl chloride and vinyl acetate which are of greatlyimproved thermal resistance to decomposition and degradation whenemployed in the priming of zinc and zinc-alloy surfaces and which areentirely suitable for use at baking temperatures higher than hithertoavailable without undesirable chemical change. Other objects of thisinvention will appear hereinafter from the following detaileddescription and the accompanying claims.

It has now been found that improved metal priming or undercoatingcompositions comprising a copolymer of vinyl chloride and vinyl acetatemodified with an unsaturated dicarboxylic acid may be obtained if saidpriming or undercoating compositions are further modified byincorporating a polyhydroxy compound therein in which said polyhydroxycompound contains at least four available hydroxy groups. Thismodification, surprisingly enough, results in the formation of primercompositions or undercoatings of substantially increased thermalstability with the result that they may be heated during bakingoperations to temperaturts of as high as 190 C. and maintained at suchtemperature for relatively long periods without any apparentdiscoloration or other change indicative of any decomposition takingplace. In accordance with the present invention, which as brought outabove involves the addition of a polyhydroxy compound to said copolymerprimer compositions, it has been found that metal surfaces comprisingzinc and its alloys, for example, surfaces which have been notoriouslydifficult to prime satisfactorily with suitable adhesion, may now becoated with polyvinyl chloride plastisols without any necessity forlimiting or otherwise altering the temperatures or the duration of thebaking treatment these coating materials require in order to effect thedesired degree of fusion or gel formation in producing the desiredsurface characteristics.

As some examples of suitable polyhydroxy compounds which may be employedin accordance with the present invention there may be mentionedpolyvinyl alcohol, pentaerythritol, and hydroxyethyl-cellulose, it beingunderstood, of course, that in the case of each of these modifyingagents they will contain at least four available hydroxy groups.

The amount of said polyhydroxy compound added to the priming compositionto obtain the desired heat resistance and stability will normallyincrease the higher the baking temperature employed and the longer theperiod of time to which the primer or undercoating composition isexposed to these temperature conditions. In addition, it has beenobserved that the amount of polyhydroxy compound necessary to producethe desired thermal stability depends also upon the composition of thealloy. Accordingly, it has been found that in order to obtain thedesired degree of thermal stability the priming compositions willcontain from about 20% to 200% by weight of said polyhydroxy compound,based upon the weight of the polyvinyl copolymer film-forming componentin said composition. Preferably, the stabilized primer composition willcontain about by weight of the polyhydroxy compound based upon theWeight of the polyvinyl copolymer present for optimum results.

These primer compositions are ordinarily formulated by dissolving theunsaturated acid modified vinyl copolymer in a volatile liquid solventmedium together with the polyhydroxy compound and then spraying, dippingor brushing the undercoat on the metal surface. The solvent employed iscommonly a mixture of an aliphatic ketone such as methyl ethyl ketone,methyl butyl or isobutyl ketone or methyl amyl ketone with an aromaticsolvent such as toluene or Xylene.

An enhanced degree of thermal stability is also obtained if not only thepriming composition but also the surface coating composition appliedover said undercoat which may be a polyvinyl chloride plastisol ororganosol, for example, is also modified by the addition of saidpolyhydroxy compound thereto. Alternatively, the coating composition mayalso be modified by incorporating therein the usual stabilizing agentssuch as the organic tin or lead compounds commonly utilized for thispurpose in th art. Bodying agents, fillers and pigments well-known inthe art may also be incorporated in the coating composition which areapplied over the primer.

It is to be understood, of course, that the novel primer compositions ofthis invention may be employed alone for protecting metal surfaceswithout any added surface finish coating being applied over the primer.This priming treatment, furthermore, is suitable for the priming of allother metal surfaces and especially those which are subsequently to beexposed to higher temperatures in a fusing or baking step and is notlimited to those situations only where a plastisol or organosol surfacecoating is subsequently applied over the undercoat and baked. Inaddition, these novel priming compositions serve admirably as a basecoating for metal surfaces upon which other bodied and pigmented coatingcompositions may be applied such as, for example, those surface coatingswhere the film-forming component is a chlorinated rubber, nitrocelluloseor an alkyd resin.

In order further to illustrate this invention but without being limitedthereto, the following examples are given:

Example 1 The surfaces of a sheet of iron are mechanically scoured andare then coated with a primer of the following composition:

Parts by weight Copolymer consisting of 84% by weight of vinyl chloride,15% by weight of vinyl acetate and 1% by weight of maleic acid 2.4Methyl ethyl ketone 16.3 Toluene 1.3 Pentaerythritol 2.0

After the volatile solvent has competely evaporated from the primercoating applied to the sheet stock, the primed surface is then coatedwith a polyvinyl chloride plastisol composition consisting of 60 partsby weight of finely divided polyvinyl chloride and 40 parts by weight ofdioctyl phthalate plasticizer. The. coated sheet thus obtained is thenbaked for 15 minutes at a temperature of 170 C. to fuse the coating andis then baked for an additional 15 minutes at 190 C. to flow out thecoating and bring it to maximum gloss. No discoloration or decompositionof the adherent primer undercoating is noted under these conditions.

Example 2 i The surfaces of a zinc plate are coated with a primer of thefollowing composition:

Parts by weight I Modified copolymer of Example 1 2.4 Methyl ethylketone 16.3 Toluene 1.3 Polyvinyl alcohol (finely-divided) 2.0

After complete evaporation of the solvent from the primer coating thusapplied the undercoated zinc plate is then Example 3 The surfaces of ascoured zinc sheet are first coated with the primer compositiondescribed in Example 2 and after the volatile solvents have completelyevaporated, the coated sheet is then baked at 180 C. for minutes.

After cooling, the primed zinc sheet is then coated with a clearnitrocellulose base lacquer. The high-bake treatment utilized is foundto leave the primer coating unaffected with respect to any apparentdiscoloration or dcomposition.

We claim:

1. A method of coating zinc and zinc-alloy surfaces which comprisespriming said zinc and zinc-alloy surfaces with a solution in a volatilesolvent medium of a film-forming terpolymer of vinyl chloride, vinylacetate and an ethylenically unsaturated aliphatic dicarboxylic acid,said terpolymer solution also containing from 20% to 200% by Weight,based on the weight of said terpolymer, of a polyhydroxy compound havingat least four available hydroxy groups and selected from the groupconsisting of polyvinyl alcohol, pentaerythritol andhydroxyethyl-cellulose, evaporating the volatile solvent medium to leavea primer coating of ,the polyhydroxy compound modified film-formingterpolymer on the zinc and Zinc-alloy surface, coating the primedsurface with a resin selected from the group consisting of polyvinylchloride, chlorinated rubber, nitrocellulose and alkyd resins, and thenbaking.

2. A method of coating zinc and zinc-alloy surfaces which comprisespriming said zinc and zinc-alloy surfaces with a solution in a volatilesolvent medium of a filmforming terpolymer of vinyl chloride, vinylacetate and maleic acid, said terpolymer solution also containing from20% to 200% by weight, based on the weight of said terpolymer, apolyhydroxy compound having at least four available hydroxy groups andselected from the group consisting of polyvinyl alcohol, pentaerythritoland hydroxyethylcellulose, evaporating the volatile solvent medium toleave a primer coating of the polyhydroxy compound modified film-formingterpolymer on the zinc and zinc-alloy surface, coating the primedsurface with a resin selected from the group consisting of polyvinylchloride, chlorinated rubber, nitrocellulose and alkyd resins, and thenbaking.

3. A. method of coating a metal surface which comprises priming saidmetal surface with a primer consisting essentially of a volatile solventsolution of a terpolymer of vinyl chloride, vinyl acetate and anethylenically unsaturated aliphatic dicarboxylic acid and from 20% to200% by weight, based on the weight of said terpolymer, of a polyhydroxycompound containing at least four hydroxy groups and selected from thegroup consisting of polyvinyl alcohol, pentaerythritol andhydroxyethylcellulose, evaporating the volatile solvent to leave saidprimer coating on said surface coating the primed surface with afilm-forming resin, and then baking.

4. The method of claim 3, wherein said film-forming resin coating alsocontains a polyhydroxy compound containing at least four hydroxy groupsselected from the group consisting of polyvinyl alcohol, pentaerythritoland hydroxyethylcellulose.

5. The method of claim 3, wherein said metal surface comprises zinc.

6. The method of claim 3, wherein said solvent solution contains aboutby weight of said polyhydroxy compound, based on the weight of saidterpolymer.

References Cited by the Examiner UNITED STATES PATENTS 2,329,456 9/1943Campbell 26078.5 2,764,313 9/1956 Foley et al ll7--75 2,891,876 6/1959Brown et al. l1775 2,913,430 11/1959 Roeser 26032.8

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

A. H. ROSENSTEIN, Assistant Examiner.

1. A METHOD OF COATING ZINC AND ZINC-ALLOY SURFACES WHICH COMPRISESPRINTING SAID ZINC AND ZINC-ALLOY SURFACES WITH A SOLUTION IN A VOLATILESOLVENT MEDIUM OF A FILM-FORMING TERPOLYMER OF VINYL CHLORIDE, VINYLACETATE AND AN ETHYLENICALLY UNSATURATED ALIPHATIC DICARBOXYLIC ACID,SAID TERPOLYMER SOLUTION ALSO CONTAINING FROM 20% TO 200% BY WEIGHT,BASED ON THE WEIGHT OF SAID TERPOLYMER, OF A POLYHYDROXY COMPOUND HAVINGAT LEAST FOUR AVAILABLE HYDROXY GROUPS AND SELECTED FROM THE GROUPCONSISTING OF POLYVINYL ALCOHOL, PENTAERYTHRITOL ANDHYDROXYETHYLCELLULOSE, EVAPORATING THE VOLATILE SOLVENT MEDIUM TO LEAVEA PRIMER COATING OF THE POLYHYDROXY COMPOUND MODIFIED FILM-FORMINGTERPOLYMER ON THE ZINC AND ZINC-ALLOY SURFACE, COATING THE PRIMEDSURFACE WITH A RESIN SELECTED FROM THE GROUP CONSISTING OF POLYVINYLCHLORIDE, CHLORINATED RUBBER, NITROCELLULOSE AND ALKYD RESINS, AND THENBAKING.